1. Field of the Invention
The present invention relates to actuators in disk drives, and to compensation for thermal expansion of the actuators. Particularly, the present invention relates to rotary actuators for quickly reading from, or writing to, a disk drive, and specifically to low inertia rotary actuators with reduced thermal misregistration throughout their operating temperature range.
2. Description of Related Art
An actuator is an assembly of components for a disk drive assembly that reads data from, and writes data to, one or more storage disks. The storage disks are typically of the magnetic type, although other types of disks can be used. For a magnetic disk drive assembly, an actuator is typically a pivoting assembly that includes an armset having two or more arms, a magnetic transducer head affixed on each of the arms, a voice coil or stepper motor for rotating the armset, and a bearing aligned with an axis about which the armset rotates. The actuator may include one or more pairs of arms. For single disk systems, the magnetic disk is positioned between a pair of arms; for multiple disk systems, each disk is positioned between one of a number of pairs of arms. The magnetic heads on the arms are positioned just off the surface of the respective disk, and on either side. The armset is rotatable to move the heads over the surface of the disk, in order to access a particular track on the disk.
Disk tracks are laid out in a series of concentric circles. Any location on the disk can be accessed by a combination of rotating the disk and moving the magnetic heads from track to track. The data stored on the disk is very dense; for example, forty megabytes (4 million bytes, each having 8 bits) can be stored on a hard disk having a diameter of only a few inches. Due to the high density of data, very precise head positioning is necessary to read a particular bit of data.
The environment in which the disk drive is positioned, such as the inside of a computer, contains many heat producing elements, and the temperature of the disk drive can increase substantially during the course of a computing session. Therefore, disk drives are usually specified for operation within a wide temperature range, such as 5.degree. to 50.degree. Centigrade.
In many prior art systems, thermal effects can shift the position of the heads off track. Thermal expansion occurs in all materials to an extent quantified by the thermal expansion coefficient of the material. Accordingly, many prior art actuators can and do expand and distort with temperature changes during the course of a computing session. Expansion can cause the heads to be misaligned with the disk tracks causing problems in reading and writing data. This problem may be termed "thermal misregistration".
In order to compensate for thermal misregistration, many systems have been developed, some mechanical or electrical in nature, and others software related. The software related compensation systems typically use one or more servo tracks recorded on the disk to provide a tracking reference. For example, Thano et al. (U.S. Pat. No. 5,005,089) discloses a system for thermal compensation of a disk drive, including a temperature sensor, two prerecorded data tracks for calibration, and software that performs the calibration with the actuator and compensates its position for thermal misregistration. The temperature sensor is periodically checked and recalibration is carried out when the drive temperature changes. As another example using multiple disks in a stacked configuration, Repphun et al. (U.S. Pat. No. 4,924,337) discloses a servo head, separate from the data heads, for reading tracking information from a middle disk. The servo information is used to make certain that the head drive assembly, or actuator, positions the heads above the center of the data track regardless of thermal effects or other factors. In general, software based systems for correcting misregistration have drawbacks including the amount of time lost sensing the misregistration, and the amount of time lost making adjustments to compensate. Furthermore, additional costs are associated with software, both in development cost, memory space, and computer time that could be better utilized.
Some prior art systems for thermal compensation include additional mechanical or electrical elements for thermal compensation. For example, Schmitz (U.S. Pat. No. 4,814,908) discloses an arm having a heating element embedded therein. The heating element is positioned so that the linear expansion or contraction of the arm resulting from the temperature rise or drop due to the amount of heat applied to the arm by the heating element will move the transducer across the track. During tracking, a circuit varies the amount of power applied to the heating element so that the transducer is kept over the center of the track. As another example, Cain (U.S. Pat. No. 4,860,135) discloses placing a slot in the edge of at least one arm and placing a body with a different thermal coefficient into the slot with an interference fit. The temperature coefficients of the body and the arm are chosen to cause temperature-induced distortion of the arm in which the body is placed. This distortion corrects at least in part the misalignment in the arm assembly. As a further example, a temperature compensating mechanism disclosed by Kobayashi et al. includes spring elements, attached at spaced locations to the actuator assembly, and associated with a cooperating pulley that drives a band. Temperature variation is compensated by proper selection of the spring coefficient of the band, spring, and connector structure between the pulley and one of the spaced locations, and the spring coefficient between the pulley and another location. As yet another example, Williams et al. (U.S. Pat. No. 4,969,058) discloses a configuration which is said to nullify the effects of thermal and hygroscopic expansion if certain structural members in a fine position actuator specified therein are constructed of a material that satisfies an equation provided therein.
Some systems attempt to address the thermal expansion issue by minimizing the temperature change or temperature gradients, thereby minimizing thermal expansion effects. An example of this approach is Iida et al. (U.S. Pat. No. 4,660,110) which discloses a magnetic disk storage device with an inner shroud having a multiplicity of small apertures for releasing heat to the outside, which are said to avoid thermal off-track of the magnetic heads. Another example is provided by Stefansky et al. (U.S. Pat. No. 4,979,062), which discloses a two-dimensional base plate. Misalignment of the heads and the disks is said to be prevented by the two-dimensional structure which has increased structural rigidity during thermal expansion over a three-dimensional structure, which flexes unpredictably. Another example is provided by O'Sullivan et al. (U.S. Pat. No. 4,980,786) which discloses multiple approaches to the problem of thermal expansion, including a carefully constructed "box within a box", and a symmetrical design with components that have thermally matched expansion coefficients.
These designs may be useful in reducing the thermal expansion problem; however, regardless of box construction, temperature changes are inevitable to some extent, unless a completely controlled environment (i.e., refrigeration) were used to maintain a constant temperature. For most uses, it is simply impractical and not cost effective to provide a such temperature controlled environment.
Symmetrical construction using thermally matched components, as disclosed in some of the above patents, is impractical for many typical disk drive components due to the properties of the different materials. In many actuator assemblies, the actuator armset is positioned to pivot about bearings that are positioned in a bore provided in the armset. The Stefansky et al. reference discloses such an arrangement. Thermal expansion matching is simply not possible in that actuator, because the bearing material has different properties than the armset material. The bearing should be made of a hard, durable material with excellent wear properties, such as steel, that can be formed to precise dimensions and case-hardened, and does not deform under stress. On the other hand, the armset should be made of a lightweight, low inertia material, such as magnesium or aluminum, that has good damping characteristics to avoid instabilities in the closed loop servo control system. A low inertia armset can move quickly to the next track for fast data access.
The different materials have different thermal expansion coefficients; steel has an expansion coefficient that is about one-half that of magnesium. As temperature increases, the dimensions of the bore increase twice as fast as the dimensions of the steel bearing. If a steel bearing were to be slip fit into the bore at room temperature, then at higher temperatures the bearing would become "sloppy", and move about as the bore's dimensions become larger than the bearing's dimensions. Steel bearings are available in an integral form, which includes the bearings positioned within a cylinder so that the cylinder rotates around the axis. The steel integral bearing requires an interference (i.e., a press) fit within the armset bore, so that it does not become loose, or slip during operation. In other words, during manufacture, the bearing must be fit very tightly into the armset bore so that when temperatures increase during operation, a tight fit is maintained. Press fitting a bearing has caused problems in manufacturing actuators; many bearings were distorting and malfunctioning as a result of being compressed too tightly. This problem has caused low yields in manufacturing.
It would be an advantage to provide an actuator with simple, reliable thermal compensation included therein, and it would be a further advantage if the actuator could be manufactured with a high yield of usable product.